1. Field of the Invention
The present invention relates to a process for producing an aluminum alloy sheet product. More particularly, the present invention relates to a continuous casting process for producing an aluminum alloy sheet product that is suitable for beverage container end stock.
2. Description of Related Art
Aluminum alloy sheet stock with a relatively high magnesium content, such as AA 5082 or AA 5182 alloy sheet stock, is used to form ends for carbonated beverage containers. The ends must have a sufficient strength since a beverage container should be able to withstand an internal pressure of at least about 60 pounds if it is to contain unpasteurized beer and at least about 90 pounds if it is to contain pasteurized beer, soda pop, or any beverage having similarly high carbonation levels.
Aluminum alloy sheet is typically produced by direct chill casting of a molten aluminum alloy into an ingot which is then rolled into a strip. Alternatively, aluminum alloy sheet may be produced by a continuous strip casting process. An apparatus for continuous strip casting using a block caster is described in U.S. Pat. Nos. 3,709,281, 3,744,545, 3,747,666, 3,759,313 and 3,774,670.
In a block casting process, molten aluminum alloy is injected through a nozzle, or distributor tip, into a cavity formed between two sets of opposed chilled blocks that are continuously moving in a direction away from the distributor tip. While in the cavity, the alloy cools and solidifies to form an aluminum sheet. The aluminum sheet then exits the block caster and passes between rollers to further reduce the thickness of the strip. This is typically referred to as hot rolling.
As the continuous strip comes out of the hot rolling step, it is coiled and allowed to cool. The coil is then cold rolled to further reduce the thickness of the strip. Often, the strip will be cold rolled in several passes with an annealing (heat treatment) step between the cold rolling passes.
The continuous strip casting process using a block caster has been shown to be effective for producing aluminum alloy sheet from low magnesium alloys, for example, alloys having a magnesium content of less than about 4 percent. As used throughout this specification, including the claims, all percentages refer to weight percent, unless otherwise noted.
For example, U.S. Pat. No. 4,260,419 by Robertson discloses the use of a continuous strip casting process to cast an aluminum alloy having from about 1.3 percent to about 2.5 percent magnesium and from about 0.4 percent to about 1.0 percent manganese. U.S. Pat. No. 5,106,429 by McAuliffe et al. discloses a process for continuously casting an aluminum alloy sheet comprising from about 2 percent to about 2.8 percent magnesium and from about 0.9 percent to about 1.6 percent manganese. However, it is not believed that is has heretofore been possible to continuously cast an aluminum alloy sheet having significantly higher magnesium contents, such as the high magnesium aluminum alloys used for beverage container ends.
Since continuous casting using a block caster is an economical method for the production of aluminum alloy sheet, it would be useful to provide a continuous casting process for the fabrication of an aluminum alloy sheet product having high levels of magnesium which can be used for container end stock.